The present invention relates to an electromagnetic rotating machine, and particularly to an electromagnetic rotating machine such as induction machines, synchronous machines, the stator of which has radial position control windings for the magnetic bearing function.
In recent years, it has been demanded that the motor speed and output be increasingly high for machine tools, turbo-molecular pumps, fly-wheels, etc. The bearings for these apparatuses are magnetic bearings, which are adaptable for high speed rotation and need no maintenance for a long time.
Such a magnetic bearing tends to be large in size to generate sufficient force, and may have a length equal to the axial length of a motor. As a result, the main shaft of the motor becomes long, so that it elastically vibrates when it rotates at high speed. It is therefore not easy to achieve high speed rotation. In addition, in order to achieve high motor power, a motor needs a longer shaft. This increases the attraction force generated by the electromagnetic machine, so that there is a need to enlarge the size of the magnetic bearings as well.
As a result, the critical speed becomes low, so that it is very difficult to make the motor speed higher.
In view of such problems, in recent years, electromagnetic rotating machines with position control windings have been developed, in which position control windings for the magnetic bearings are added to the stators of the motors, so that the shaft length is short and the speed and output are high.
FIG. 7 of the accompanying drawings shows an electromagnetic rotating machine 30 having position control windings. The machine 30 includes a rotor 31, stators 32 and 84, and three-phase inverters 20, 36 and 38.
The stators 32 and 34 of the motor have windings (not shown) for generating torque, which are connected with the inverter 20. The stators also have radial position control windings (not shown) for generating radial magnetic force for the rotor 31. The inverters 36 and 38 control the currents of the radial position control windings of the stators 32 and 34, respectively, to regulate radial positions of the rotor shaft 31. The electromagnetic rotating machine 30 has position control windings which can generate both torque and radial force with one stator. The machine 30 can be short in axial length compared with conventional high speed motors with magnetic bearings. If the axial shaft length is the same as the conventional machine the speed and output of the motor 30 can be high.
Electromagnetic rotating machines with position control windings have already suggested can be reviewed as follows.
In some disc type motors, axial force is generated by varying the exciting magnetic fluxes, so that the axial position of each rotor is adjusted. This can be applied to disc type rotating machines, but this is difficult to apply to radial rotating machines widely used.
In some general induction motors, the windings can be split and currents applied thereto are unbalanced to generate radial force for radial position control. When the rotor is positioned centrally, however, it is theoretically not possible to generate radial force.
Japanese Patent First (before exam.) Publication S.64-55,031 discloses magnetic paths simply common to magnetic bearings and a stepping motor. This method is suitable for low speed actuators, but is not suitable structurally for very high speed rotation since it is necessary to make the number of poles very high. It is difficult to apply this method to rotating machines having sinusoidal magnetomotive force distribution and/or magnetic flux distribution, which are widely used for high output induction machines, permanent magnet type motors, etc.
Japanese Patent First Publication H.4-2386,188 discloses art in which the number of poles is reduced, and suggests structure similar to conventional induction machines and permanent magnet type rotating machines. In this art, the stator is similar to the stator core of a four-pole switched reluctance machine. The stator has eight teeth with four-pole concentrated windings, which are divided with the magnetic poles so that the magnetic flux of each pole is independently controlled. It is possible to generate a rotating magnetic field by varying the magnitude of the flux of each pole. It is also possible to generate radial force as in conventional magnetic bearings.
Japanese Patent First Publication H.4-107,318 discloses similar core structure, which is characterized by distributed windings for magnetomotive force distribution close to sinusoidal distribution.
In the art of the last two publications, the windings are divided into four and are independently driven. If the windings are two-phase, there is a need for at least eight single-phase inverters and 16 wires in one unit for generating radial forces in two orthogonal axes as well as torque. In addition, the radial forces and the torque are controlled with the same winding current, so that there is a need for a current driver of very high speed, high precision and large capacity.
Japanese Patent First Publication H.2-193,547 discloses a four-pole electromagnetic rotating machine, which is provided with two-pole windings to generate radial force. This machine may be a rotating-field type motor, in which, by adding position control windings each having a different number of poles to the stator, the rotating magnetic field is positively unbalanced to generate radial force together with torque.
FIG. 8 shows the principle of generation of radial force in an electromagnetic rotating machine of this type. The machine includes a rotor 40 and a stator 42, which has four-pole windings 44 for generating torque.
If the rotor 40 is positioned coaxially with the stator 42, the windings 44 generate four-pole symmetrical magnetic fluxes H4 by being supplied with positive current. A four-pole rotating magnetic field is generated by supplying two-phase alternating currents to the four-pole windings 44 and the other four-pole windings (not shown) perpendicular to them. As disclosed in Japanese Patent First Publication H.2-193,547, the windings may otherwise be three-phase. Thereby, if the rotor 40 has a cage winding, torque is generated in the rotor as the rotor of an ordinary induction machine.
The stator 42 also has two-pole position control windings 46a and 46b, in addition to the four-pole windings 44. The windings 46a and 46b make magnetic force act radially on the rotor 40. When the winding 46a is supplied with positive current, two-pole magnetic fluxes H2 are generated as shown in FIG. 8.
In this case, at the air-gap under the rotor 40 (in FIG. 8), the direction of the four-pole fluxes H4 is opposite to that of the two-pole fluxes H2, so that the flux density decreases. On the other hand, at the air-gap over the rotor 40, the directions of the fluxes H4 and H2 coincide, the flux density increases.
The flux distribution is thus unbalanced, which causes a radial force F in the rotor 40, which acts upward in FIG. 8 so as to levitate the rotor within the stators. The force F can be adjusted by controlling the magnitude of the current flowing in the two-pole winding 46a. The direction of the force F can be reversed by reversing the direction of the current in the winding 46a.
It is possible to generate radial force in the right or left direction in FIG. 8 by supplying current to the two-pole winding 46b, which is perpendicular to the winding 46a. By adjusting the magnitude and directions of the currents in the two-pole windings 46a and 46b, it is possible to generate a radial force in the desired magnitude and direction.
In FIG. 8, the four-pole windings 44 are used to drive the motor, while the two-pole windings 46a and 46b are used for radial position control. Otherwise, the four-pole windings 44 may be used to generate radial force, while the two-pole windings 46a and 46b may be used to drive the motor.
In an electromagnetic rotating machine which provides control of the rotor in accordance with a principle as stated above, and which has three-phase windings, it is possible to generate radial force and torque with only six wires and two three-phase inverters. Because the windings for generating radial force are separate from those for generating torque, it is possible to use an inverter or a power amplifier of small power capacity for radial force control. Because the four-pole and two-pole windings are used, the mutual couplings are zero if the rotor is positioned centrally, and no induced voltage of the motor is generated in the radial force control windings. Such electromagnetic rotating machines can be widely used as high output rotating machines such as induction machines, permanent magnet type synchronous machines, synchronous reluctance motors,and etc., in which sine-wave magnetomotive force distribution and/or sine-wave magnetic flux distribution are/is estimated.
FIG. 9 shows the control system of an electromagnetic rotating machine with position control windings. To simplify the setup, FIG. 9 shows only the system setup for two-pole radial position control.
The control system shown in FIG. 9 consists essentially of a motor drive (circuitry) section A for applying torque to the rotor 50, and a position control (circuitry) section B for controlling the radial position of the rotor 50.
In the motor drive section A, the sine-wave oscillator 52 and cosine-wave oscillator 53 generate sine and cosine waves, respectively, based on the primary current frequency command 2.omega.. The waves are multiplied by a current amplitude command value. The amplitude and phase of the current as well as the frequency are determined by motor drive controller (not shown) depending on motor/generator types. In the case of induction motors,field oriented controllers can be used. Vector controllers can be used in other types of electric machines. Simple v/f controllers can be used in accordance with loop gain adjustment of radial position control loops together with main flux amplitide and rotational position estimater.
The two-phase/three-phase converter 54 makes two-phase/three-phase conversion and outputs instantaneous current command values iu4, iv4 and iw4. In accordance with these values, the inverter 56 controls the currents of the four-pole motor windings C4.
The air-gap lengths of the rotor 50 are detected by displacement sensors 58a and 58b.
In the position control section B, the adders 60a and 60b compare the outputs .alpha. and .beta. from the displacement sensors 58a and 58b with gap command values .alpha.0 and .beta.0, respectively. The errors .epsilon..alpha. and .epsilon..beta. are supplied to the controllers such as PID (proportional integration/differentiation) 62a and 62b, respectively. The controllers 62a and 62b generate command values F.alpha. and F.beta., respectively, which command the radial forces in accordance with the air-gap errors .epsilon..alpha. and .epsilon..beta.. The modulator 64 modulates the command values F.alpha. and F.beta. on the basis of the signals from the sine-wave oscillator 52 and cosine-wave oscillator 53, respectively, in accordance with the angle of the rotating magnetic field, which may vary depending on load conditions,and generates current command values i.alpha. and i.beta. for the two-pole position control windings N.alpha. and N.beta.. If the rotor has windings of squirrel cage type, a compensator 66 is provided for phase compensation, as shown in FIG. 9. The two-phase/three-phase converter 68 converts the output signals of the compensator 66 in two-phase axes into values in three-phase axes. In accordance with the current command values iu2, iv2 and iw2 from the converter 68, the inverter 70 regulates the winding currents Iu2, Iv2 and Iw2 of the two-pole position control windings N.alpha. and N.beta..
In the motor drive section A, the manner in which to generate frequency command values and current amplitude command values depends on the type of motors and control methods such as vector control, slip frequency control, and v/f control as well as constant id control. For vector control etc., the motor drive section A supplies the position control section B with the amplitude and angle of the field and etc., which are omitted from FIG. 9, in addition to the sine and cosine waves of the frequency. These commands can be generated based on detected shaft speeds,rotational angles and motor currents.
As described above, the control system shown in FIG. 9 is very similar to the control systems of the conventional magnetic bearings, except for modulation in synchronism with the rotating magnetic field.
The conventional electromagnetic rotating machines with position control windings, however, have problems with the displacement sensors for detecting the displacements of their respective rotors. Specifically, since the displacement sensors are not necessarily mounted where radial force is generated, elastically deformable rotors may cause instability. In addition, eccentricity and roughness of the sensor target surface, etc. may cause disturbance of the position control systems. It is another problem that displacement sensors themselves are expensive.
In order to solve the above problems with displacement sensors, it is contemplated that a magnetic bearing is made sensorless by the following methods.
One of the methods is to estimate the radial position variation of the main shaft from the switching frequency of a current controller for driving a magnetic bearing, because the switching frequency changes with the inductance input from the magnetic bearing terminals. This method is resistant to noise and practical. In this method, however, even if the rotor is displaced, the inductance of the magnetic bearing is not very largely changed. In particular, the inductance variation is very small near the center if differential windings are employed. The control is therefore difficult.
In another of the methods, the relation between the voltage and current of a magnetic bearing is a function of the distance between the bearing and the associated main shaft, linearization is made near the operating point, and radial displacement is estimated with an observation apparatus or a simulator, or a controller is set up for stabilizing the whole system from the detected current to generate a voltage command value. This method is mathematically very clear, and new. It is, however, necessary to make linearization near the operating point. It is also very difficult to adjust parameters to set up a stabilizing controller.